hw/ppc/ppc405_uc: Convert away from old_mmio

hw/ppc/ppc405_uc: Convert away from old_mmio

Convert the devices in ppc405_uc away from using the old_mmio
MemoryRegion accessors:

 * opba's 32-bit and 16-bit accessors were just calling the
   8-bit accessors and assembling a big-endian order number,
   which we can do by setting the .impl.max_access_size to 1
   and the endianness to DEVICE_BIG_ENDIAN, and letting the
   core memory code do the assembly
 * ppc405_gpio's accessors were all just stubs
 * ppc4xx_gpt's 8-bit and 16-bit accessors were treating the
   access as invalid, which we can do by setting the
   .valid.min_access_size and .valid.max_access_size fields

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>